Sensor arrangement with read-out means for difference generation

ABSTRACT

A sensor device with a number of sensor elements is read by a read-out unit, such that, on various partial measurements, varying difference values can be generated by adding and subtracting the measured values from varying sensor elements.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to GermanApplication No. 02/04428 filed on Mar. 12, 2002, the contents of whichare hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a sensor device having a number of sensorelements by which measurements can be made, and having a read-out unitfor reading out the measurements from the sensor elements. In addition,the invention relates to a method for reading sensor elements.

2. Description of the Related Art

Sensor arrangements having a number of sensor elements are known fromelectronic cameras for example. In this case an image is projected ontoa CCD (charge coupled device) via a lens system. The image is recordedby the CCD based on a measurement.

Owing to the large number of sensor elements on a CCD, the imagerecorded in this way requires a very large amount of memory and also avery large amount of transmission capacity when transferring the imagefrom the camera for further processing.

Hence it is known in the art to apply a compression technique to theimage. Wavelet transformations with subsequent compression have becomeestablished in particular for this purpose, because they lead to a veryhigh compression rate and a large part of the image content can still bereconstructed even when only part of the data has been transferred.

The wavelet transformations do not make the memory in the cameraobsolete, however, because the image still needs to be storedtemporarily prior to the transformation. Furthermore, an additionalprocessor unit is required in order to perform the transformation. Thismakes the camera more expensive and increases its power consumption.

Hence an object of the invention is to record images in such a way thatan easily compressible representation of the image content is obtainedback at the recording stage and an additional processor unit is notrequired.

SUMMARY OF THE INVENTION

According to the present invention, the sensor device has a number ofsensor elements by which measurements can be made. In addition, thesensor device has a read-out unit for reading out the measurements fromthese sensor elements. The sensor device is designed so that to performan overall measurement, a plurality of partial measurements are made insuccession. The term partial measurements refers here to the process inwhich a plurality of partial measurements are made successively overtime that together produce the measurements for the required overallmeasurement. Thus the partial measurements are each made by involving,at least approximately, the same, in particular all the sensor elements.

The read-out unit is designed in order to switch the reading of thesensor elements in such a way that for the respective partialmeasurements the measurements of the different sensor elements are eachadded and subtracted for difference generation. In this way, for eachpartial measurement, absolute values are no longer measured for theindividual sensor elements but just difference values between themeasurements of the sensor elements, which are obtained from thecombinations of sensor elements defined for each partial measurement. Inaddition, a different difference is generated in each partialmeasurement in order to obtain the required information, for example animage, for the overall measurement.

Thus for the sensor arrangement, one starts from the same idea as thewavelet transformation already known in the art for post-processing. Inboth cases difference values are found primarily by combining imagepixels. This is done in the knowledge that in an image the differencebetween two pixels often vanishes, so that these vanishing differencescan together be compressed or left out completely.

Unlike a wavelet transformation performed subsequently, however, in thesensor device the sensor elements are read out immediately by theread-out unit in such a way that the read-out result contains differencevalues and no absolute values from the outset. This means that aprocessor for post processing is not required.

It is crucial here, how the read-out unit switches the reading of thesensor elements for the respective partial measurements, that is, howthey combine the sensor elements positively and negatively in each caseso that one can construct the overall measurement as completely aspossible from the results of the partial measurements. In addition, onlyas few partial measurements as possible are to be made and/or eachpartial measurement should be made as quickly as possible.

These requirements are met by combining the sensor elements for eachpartial measurement positively and negatively in such a way that in eachcase they produce the basis vector of a basis in which the result of theoverall measurement can be represented. In this way one obtains noredundant information and thus one only needs a minimum number ofmeasurements, while on the other hand no information is lost.

Preferably the basis is the basis of a wavelet transformation. Inparticular, this may be the Haar basis because this has proved to beparticularly easy to implement in circuit technology.

Alternatively, however, prefactors can also be applied to themeasurements of at least some of the sensor elements when adding andsubtracting. It is thereby possible, for example, to use the Daubechiesand/or the Gabor basis as basis.

The sensor elements are preferably arranged so as to record spatiallydistributed measurements, in particular an image. This can be done, forexample, by arranging the sensor elements in a plane, if one provides anoptical system, for example made of lenses, that projects the image tobe recorded onto the sensor elements arranged in the plane.

The sensor device can be produced particularly economically if thesensor elements are sensor elements of a CCD.

The number of sensor elements is preferably greater than or equal to 16.For particularly simple switching, the number should be 2^(n), and 4^(n)in the two-dimensional case, where n is a natural number.

The number of partial measurements is greater than or equal to 3, inparticular greater than or equal to 5.

For the difference generation, the number of measurements that are addedin the partial measurements is at least approximately equal to thenumber of measurements that are subtracted. Thus a vanishing differenceresults if no differences in intensity occur in the image.

During the partial measurements, sensor elements and hence themeasurements of these sensor elements, are combined in groups in whichthe measurements of the different sensor elements for the group are eachconnected together additively and subtractively. The combination ofsensor elements into groups is thus advantageous because by selecting asuitable basis, in particular a wavelet basis, the measurements for theindividual groups can be de-coupled from each other. This means that theresults of the partial measurements are independent of each other forthe individual groups, so that the partial measurements can also beperformed independently of each other. The result of the overallmeasurement is no longer dependent on a relationship between the partialmeasurements.

As already mentioned above, the sensor device is particularly usefulwhere the differences of the added and subtracted measurements of thesensor elements vanish in many cases. If one can perform the partialmeasurements in a sequence in which the differences must get eversmaller, then the sequence of partial measurements can be terminatedwhen the result of the adding and subtracting, i.e. the differencevalue, lies below a given threshold value for a partial measurement. Onethen knows that future measurements will only produce results whoseinformation content is so low that it is no longer of interest.

Often one can specify a sequence of partial measurements in which thedifferences must get ever smaller, but which cannot be specified inadvance. Thus a termination criterion for the sequence of partialmeasurements that has proved very suitable in practice is to ceasefurther partial measurements once a required resolution is reached. Forthis purpose it is useful to order the partial measurements such thatpartial measurements that deliver coarse information, and hence onlyrequire a short exposure time, are performed first, while partialmeasurements that measure detailed information and hence take longer,are performed later or not even performed at all after reaching therequired resolution.

The required resolution for which no further partial measurements areperformed after it is reached, can in particular be adjustable by theuser of the sensor arrangement. Based on this adjustment, the systemthen determines how many partial measurements are to be performed.

The time required for recording an overall measurement can besignificantly reduced by this premature termination of the measurements.This is obviously advantageous in the general case, but particularly soif the object of which the image is to be recorded may be exposed to theradiation required for the recording. This is the case in X-rayphotographs, for example, but also for normal recordings oflight-sensitive objects. Here the radiation required for the recordingcan be switched off as soon as the required recording quality isreached.

For the case in which sensor elements are grouped together, the sequenceof partial measurements can also be terminated individually for each ofthe respective groups. This results in a reduction in the memoryrequirement, because for groups for which differences are no longergenerated there is also no need to store these differences.

In a method to read a number of sensor elements by which measurementscan be made, a plurality of partial measurements are made in successionusing read-out unit for reading out the measurements from the sensorelements in order to perform the overall measurement, where the sensorelements are switched by the read-out unit in such a way that for therespective partial measurements the measurements for different partialmeasurements from different sensor elements are each added andsubtracted.

Advantageous embodiments of the method follow analogously from theadvantageous embodiments of the sensor arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome more apparent and more readily appreciated from the followingdescription of the exemplary embodiments taken in conjunction with theaccompanying drawings of which:

FIG. 1A is a plan view of sensor elements in a one-dimensional sensorarrangement;

FIG. 1B is a switching diagram for the switching of the sensor elementsby a read-out unit in different partial measurements;

FIGS. 2 to 4 are timing diagrams for the switching over time of sensorelements of the one-dimensional sensor device of FIG. 1 combined ingroups;

FIG. 5 is a circuit diagram of a two-dimensional sensor arrangement, and

FIG. 6 is a block diagram of a sensor device in the general case.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout.

FIG. 1A shows a sensor device having sensor elements 1 to 16 arrangedone-dimensionally along a line. Although a main application area of thesensor device is in image recording, for which a two-dimensional deviceof sensor elements in one plane is appropriate, essential aspects of theinvention still arise in the one-dimensional case, so this is explainedfirst to simplify understanding.

The sensor elements 1 to 16 are arranged side by side and measureelectromagnetic radiation 17 in the form of light falling onto thesensor elements 1 to 16.

Below the sensor elements 1 to 16, FIG. 1B shows schematically how theread-out unit reads the sensor elements 1 to 16 in the partialmeasurements A to D. In this process, half of the sensor elements arealways connected positively, i.e. added; and the other half of thesensor elements connected negatively, i.e. subtracted. In the firstpartial measurement A, the area of the sensor elements 1 to 16 isdivided once in the center. In the exemplary embodiment shown, themeasurements of the sensor elements 1 to 8 of the left half are added,and the measurements of the sensor elements 9 to 16 of the right halfare subtracted. This is done by connecting the sensor elements inparallel in each of the two halves. In total a single difference valueI₀ ⁰ is obtained for the partial measurement A.

In the second partial measurement B, the sensor elements 1 to 8connected positively in partial measurement A, and the sensor elements 9to 16 connected negatively in partial measurement A, are themselves eachdivided in the center, so that the measurements belonging to one half ofthe measurements read out with the same sign in partial measurement Aare added, and the measurements belonging to the other half of themeasurements read out with the same sign in partial measurement A aresubtracted.

In total, two groups are obtained in the second partial measurement B,for each of which its own difference value is generated. This leads totwo difference values I₀ ¹ and I₁ ¹. They result from the adding andsubtracting in each group and represent the intensity difference in eachcase for their group.

In the third partial measurement C, each of the two groups from thepartial measurement B are again divided in the center, so that fourgroups of sensor elements result. Here as well, a difference value isagain generated for each group at read-out by reading out positively themeasurements from one half of the sensor elements belonging to thatparticular group, while reading out negatively the measurementsbelonging to the other half of the sensor elements of that group. Thusfour difference values are obtained for the four groups.

Further partial measurements are made subsequent to the proceduredescribed, in which the groups continue to be halved with respect to theprevious partial measurement for each partial measurement, so that thenumber of difference values read out constantly increases for laterpartial measurements, i.e. more detailed information about the image tobe recorded is provided. A recursive structure to the partialmeasurements hence results.

The maximum number of partial measurements that can be made is when alldirectly adjacent sensor elements are each alternately added andsubtracted, i.e. read out positively and negatively.

In practice, however, the measurement can be stopped early by group orin total, namely when a required level of resolution is reached.

The maximum measurement time per partial measurement is also inverselyproportional to the area connected together in each case. This meansthat for the first partial measurement A, only a very short exposure isrequired, which increases for the later partial measurements. Thus timecan be saved on the overall measurement by prompt termination of thesequence of partial measurements when a required resolution is reached.This is done by starting with the partial measurement A, which requiresthe shortest measurement time, and then ordering the partialmeasurements so that the measurement times increase in the order of thepartial measurements. A more general and even more important time savingin the context of the invention, however, results from the fact that thepartial measurements for the individual groups decouple, i.e. becomeindependent of each other, when a suitable basis is chosen. This effectis explained in detail below with reference to FIG. 4.

Overall the maximum measurement times are summed according to theharmonic series, so that the maximum overall measurement time remainsequal to the measurement time that would be required for a conventionalrecording. The switching times needed to add and subtract the sensorelements in the manner explained are also added onto this time however.

For a normal image recording it is sufficient to start with a partialmeasurement A as in the exemplary embodiment described, in which themeasurements of the one half of sensor elements 1 to 8 are added byconnecting in parallel, and the measurements of the other half of sensorelements 9 to 16 are subtracted by connecting in parallel with theopposite polarity. In normal image recordings it is in fact onlyrelative brightness levels that matter. If, however, an absolutemeasurement is to be made, then an additional partial measurement needsto be switched in before or afterwards, in which the measurements of allthe sensors are added together, thus measuring the total intensity ofthe image to be recorded.

The measurement times for the sensor elements 1 to 16 are shown in FIG.2 in an example measurement. The measurement times are obtained from theformulaΔt _(j) ^(i) =C·(I _(j) ^(i))⁻¹.

FIGS. 2 to 4 show the measurement times for the groups of sensorelements 1 to n for the respective partial measurements. FIG. 2 showsthe course of measurements over time for the individual groups when themaximum measurement time is always required for each partialmeasurement.

In practice, however, the measurement times for the partial measurementsreduce when these are terminated after a sufficient time period. This isshown in FIG. 3. To do this, the required measurement times for each ofthe partial measurements are found by a measurement-time measurement.For the case where the sensor device is a CCD arranged in a camera, thisis done according to the following principle for example: the lightincident on the CCD generates a current that charges the capacitancescontained in the CCD. The degree of charge of the capacitances ismeasured in a defined timing cycle. When the capacitances of a group ina partial measurement are sufficiently charged, the measurement time forthis group is stopped and the partial measurement for the groupterminated. This results in the reductions in the overall measurementtime shown in FIG. 3.

The procedure illustrated in FIG. 4 now also makes use of the effectthat the partial measurements for the individual groups decouple, i.e.become independent of each other, when a suitable basis is chosen. Forthis reason, one does not need to wait before starting the partialmeasurement of a group until all the partial measurements have finishedthat are one level up in the hierarchy of the recursive series ofpartial measurements than the partial measurement to be made, but it issufficient at each point in time, and for each partial measurement of agroup, if the partial measurement has finished for the group from whichthe group to be measured in the recursive sequence originates. Henceeach of the partial measurements can generally be started earlier, andone obtains the desynchronized, interleaved sequence shown in FIG. 4,which leads to a considerable reduction in the overall measurement time.

FIG. 5 shows a circuit diagram for four groups of sensor elements withread-out unit. In order to generate a complete Haar wavelet basis, thefollowing measurements are read out in the form of output voltages insuccessive partial measurements:I₀ =i ₁ +i ₂ +i ₃ +i ₄,I₁ =i ₁ +i ₂ −i ₃ −i ₄,I₂ =i ₁ −i ₂ +i ₃ −i ₄,I₃ =i ₁ −i ₂ −i ₃ +i ₄,where I_(m), m=0, . . . 3, is the difference value of the partialmeasurement m, i.e. gives the value of the basis vector measured for therespective partial measurement m, and i_(n), n=1, . . . , n, is themeasurement of the group n of sensor elements.

FIG. 6 shows in a block diagram the basic design of the sensorarrangement. The sensor device contains a clock oscillator 40 thatdefines the timing of a control unit 41 that controls a switch panel 42in order to read the sensor elements 43. Control unit 41 and switchpanel 42 are part of the read-out unit. The difference values obtainedare saved in a memory 44 in the form of a DRAM.

In the exemplary embodiments shown so far, the measurements from thesensor elements are added and subtracted in such a way that thedifference values obtained gave the values of the basis vectors of theHaar basis of a wavelet transformation. Alternatively, the measurementsfrom the sensor elements may also be added and subtracted together so asto obtain the values of the basis vectors of a Daubechies, Gabor and/orother basis. The circuitry is more complicated in this case, and themeasurements cannot be added and subtracted directly, but need to begiven prefactors at read-out. In their favor, the Daubechies and/orGabor bases are particularly suited to many image processingapplications, and they can be used to achieve a higher compression ratethan is possible with the Haar basis.

The exemplary embodiments described are suitable for making an intensitymeasurement. If a color measurement is to be made, then three recordingsfor the three primary colors can be made in the way known in the art, orthe color may be separated in another way. There is absolutely no needto use the same basis and resolution for each color here.

The sensor device and the associated method are suitable in particularfor the production of x-ray photographs, for remote sensing andastrophysics applications and for digital photography.

In general, the following advantages are obtained:

-   -   The total measurement time need not be set in advance. The        maximum resolution can always be obtained by a sufficiently long        measurement time. Overexposure effects do not arise here.    -   The measurement time can be optimized during exposure according        to the location.    -   A low-resolution image, which means a very short recording time,        can be subsequently further exposed locally without loss.    -   Theoretically the recording rate can be as fast as required. The        same hardware can be used for different environments.    -   The image information is present in compact form, in particular        in the form of wavelet coefficients. This means a low memory        requirement and a good basis for further data processing such as        compression.    -   Where blurring occurs during part of the measurement time or        where recording has stopped prematurely, the image is never        completely unusable.    -   A drastically reduced measurement time is obtained if only a low        resolution is required.

If the read-out unit is designed so that the read-out of themeasurements from the sensor elements can be controlled according to theapplication by suitable programming, the (wavelet) basis that suits theapplication can be applied using a sensor device that does not change inthe hardware.

The invention has been described in detail with particular reference topreferred embodiments thereof and examples, but it will be understoodthat variations and modifications can be effected within the spirit andscope of the invention.

1-14. (cancelled).
 15. A sensor device to perform an overallmeasurement, comprising: sensor elements making measurements; and aread-out unit reading partial measurements from said sensor elements insuccession by at least one of adding and subtracting measurements ofdifferent sensor elements.
 16. The sensor device as claimed in claim 15,wherein by at least one of adding and subtracting the measurements ofthe different sensor elements to obtain the partial measurements, valuesof basis vectors are obtained for a basis in which the overallmeasurement can be represented.
 17. The sensor device as claimed inclaim 16, wherein the basis is the basis of a wavelet transformation.18. The sensor device as claimed in claim 17, wherein the basis is aHaar basis.
 19. The sensor device as claimed in claim 18, wherein themeasurements of at least some of the sensor elements are givenprefactors when adding and subtracting.
 20. The sensor device as claimedin claim 19, wherein the basis is one of a Daubechies and a Gabor basis.21. The sensor device as claimed in claim 20, wherein said sensorelements record spatially distributed measurements of an image.
 22. Thesensor device as claimed in claim 21, wherein said sensor elements arein a charge coupled device.
 23. The sensor device as claimed in claim22, wherein the sensor device includes at least 16 of said sensorelements.
 24. The sensor device as claimed in claim 23, wherein saidread-out unit makes at least 3 partial measurements.
 25. The sensordevice as claimed in claim 24, wherein said read-out unit adds a firstnumber of measurements approximately equal to a second number ofmeasurements that are subtracted.
 26. The sensor device as claimed inclaim 24, wherein said read-out unit obtains the partial measurements byforming groups of said sensor elements in which the measurements ofgrouped sensor elements are added and subtracted.
 27. The sensor deviceas claimed in claim 26, wherein said read-out unit stops obtainingpartial measurements once a required resolution is reached.
 28. A methodfor reading out measurements from sensor elements to obtain an overallmeasurement, comprising: performing successive partial measurements inwhich the measurements from different sensor elements are added andsubtracted.